EEVblog 1476 - Keithley 515A Wheatstone Bridge TEARDOWN & TUTORIAL 

50 year old capacitors, probably a little leaky, but it still works well. A bit of work and you can use it again properly. Date on the board is 1967 January, but looks like the meter movement was made 1968, so probably this one dates from then, though I can guess a lot of the precision and high value resistors date from the early version period. There they used pentodes and abused the screen as the second grid for the first stage, but they used similar voltages, and the second transformer was a switched power supply. AC output socket to power the optional high voltage supply to give 500V to use the top ranges, none of this 2 power cord rubbish. My bet inside the can is a dual Jfet in the classical opamp circuit, specially selected matched pair with ultra low leakage, and all select on test resistors around this input.

That PSU was obviously made for the Aussie market. Inverted components, so that operation in the Antipodes will prevent the expected loss of electrons due to gravitational effects.

I love how top gear of the 60s and 70s was made... Straightforward and to the point with no shortcuts, and no fluf. Right to Repair was honored, even expected, with full service information supplied with purchase. Nice! Wish I owned this fine gadget.

I just realized, dialing in the resistance to find the value of the resistor is exactly the same as using gauge blocks to measure the length of something.

Looking at my small 80's era portable wheatstone bridge, looks at Dave's video, looks back at portable bridge and chucks it over shoulder..............hummphh!

The high value resistors and the meter movement had date codes, ranging from '68 to '71. The "penetrators" on that rotary switch were actually the switch contacts; flat side contact, pointy side solder terminal.

A neat trick with crappy sensors is to put a pair of them in the legs of a bridge, with only one exposed to what is being sensed. At the start of my career I used this approach to quickly evaluate "novel physics" sensors, where the rules were: "To a first approximation, a Novel Physics sensor is a..." Where what would follow would be 1) "thermometer", 2) "microphone", 3) "hydrometer", and so on. Such sensors can have the "signal of interest" buried under multiple decades of "stuff", the goal of sensor development being to determine if the desired signal can be cleanly and reliably isolated and used. The hard part was at the very start, simply finding the nominal sensor operational characteristics, starting with voltage and current bias, which needed wide-ranging bridges. We'd use nulling meters to start, then electrometers/VTVMs to obtain precise nulls. What worked in the physicists' lab often failed when moved to my bench, meaning there was lots of starting over, sometimes resulting in my part of the project being put on hold while the physicists took another stab at it. If a candidate sensor passed the "quick & dirty" bridge test, we'd go on to validate and calibrate the performance via a massive amount of data acquisition and statistical analysis (SVD, PCA, etc.), the end result of which would yield a sensor driving and compensation circuit along with a calibration procedure. My job started with working with the physicists in their lab, assisting their development of the "proof of concept" lab bench setup, done in their shielded room on an optical bench next to racks of lab equipment. I next had to replicate that system performance (as best I could) in a prototype enclosure on my engineering lab bench. I was the software/firmware engineer, but I previously worked as a tech on nuclear instrumentation and lab calibration, so I had the best precision assembly and solder qualifications in the building, and good metrology skills. I miss those days.

Nice. Reminds me of our old Schering Bridges. The hands-on method with the dials helps tremendously with understanding what you're doing. Nowadays you press a button and get a result.

I have to admire the quality of construction of this fine old piece of kit.

Bridges are so cool, I have a whole collection. Most from General Radio but also ESI and Wayne Kerr. My HP generates 1000V and can measure peta-ohm and I have a GR bridge that can generate 300V and has a zero meter that is 1 uV full scale. It can measure down to aF resolution. It once was the mother capacitance bridge from GR 's own cal-lab. If you want to know more about bridges, Henry Hall is the man that designed them for GR during 40 years. He is still active, I think he is by now in the 90's. You can reach him via the GR group or via IET. He has scanned all his GR documentation. You can find that at the IET site. Your Keithley is a very nice instrument.

The brand of bridge we had at school in the '60s was Wayne Kerr. You can imagine the ribaldry from us 15-year-old boys!

A true thing of beauty and a joy for ever indeed! And that's no fluke. Absolutely loving it, Soooooooo empty inside, but these ceramic standoffs and wafer switches surely are a looker. Wire looming too; that's the way I laced them wires in my amps since mid 2000s. I love the way the old gear was built... tube or not. BTW, one of my tech tips if anyone is interested: A Wheatstone bridge circuit and principle of operation can be used for testing ganged potentiometers (most often, stereo volume/tone pots). You just connect both ends of a DMM or even better, a positive/negative analog voltmeter to the wipers, and wire the resistive paths of both sections in parallel. Connect power (e.g. 12VDC wall wart) between left and right ends. A ganged pot with ideally consistent sections will measure zero throughout the entire length/rotation range. Typical pots tend to have inconsistencies, especially close to the extreme positions, leading to imbalance between channels. Stepped attenuators built with high precision resistors have this problem solved by design.

I like these rotary switches. Real 60es when I was born.

This belongs in a museum Dave, not in anyone's lab. Even in the 70s this was highly specialized. It's a work of art. Equipment like this was used to make the 1969 Moon Landing possible.

Test equipment in use, whiteboard theory, and a tear-down, this one's got it all!

Where art meets engineering! Always great to see this level of craftsmanship!

Potter-Brumfield pots. Nice! I used to use Bradley and P-B back in the day.

The soldering is flawless so is the engineering and the overall assembly. What a beauty.

Physics lessons in the 60s about resistors. An instrument consisting of a meter long etched rule with a resistance wire and a knife edged brass pointer a galvanometer (centre pointing ammeter) a known resistor and the DUT. Effectively the wiper being South. Junction between DUT & Known North. That takes me back.

When the numerator is 5, you don't need the Yankee allen keys. 5/64" is close enough to 2mm that they don't produce separate tools for them. The same applies to 5/32" = 4mm and 5/16" = 8mm

@24:24 the same arrangement is used for the 0-10 adjustment on the Keithley 225 current source (which I know you also own), BUT it is a plastic shaft in that onto a switch... which is just asking to break. @eevblog

The format with the whiteboard explaining stuff is brilliant and very entertaining. Love it.

Still using a 1970s Marconi TF1313A LCR meter - joy forever!

I recently got interested in electronics, I bought a piece of equipment from a shop cheap because it looked cool and what better way to lean than to tinker with stuff, no manual and not a whole lot of info on the web but I haven't done an exhaustive search, RFL 829D ac/dc calibration standard. After watching this I know a little more about how it works, seeing the big transformers in my unit and the huge red jewel light "High Voltage" and the dial below going up to 2000 volts, I'll wait till I get more info or find a manual before I plug it in ⚡🤯

i asked you on twitter about what is it and what does it do. now i got the 30 minutes video on it. when it comes to electronic you are my favourite person in it

That's a beautiful assembled piece of kit!

Those RCA transistors from that era always have a date code, in this case "2F". Last digit of the year + month (skipping letter I). So June 1972.

1968. The year I was born. Off to the moon with some really old stuff. It worked.

I believe this is the first time I saw some weird resistors in a glass tube. This is simply beautiful. Are there modern versions or alternatives to these things?

A truly wonderful piece of measuring equipment (Ah, those were the days) 👌🙂👍 ❗

good explanation Dave

Love it, thanks Dave. I have a beautiful AVO bridge and it has a very satisfying clunking display with gigantic knobs. It feels the most trustworthy device for low value capacitance, and my only truly reliable inductance meter, so I do tend to use it. It is also manageable being half as big as the Keithley! I am as enthusiastic as you with these wonders. I'd love to see the valve version.

Omg it's beautiful. Thank you Dave! I'm gonna be dreaming up my own now haha

I don't watch much of your stuff any more as it's mostly above my head being a nuts and bolts engineer...and then you go into great depth explaining the weatstone bridge which I learned doing 101 electronics, makes me wonder who your videos are aimed at.

Science Museum, London -- not only do they have a Whestone Bridge, they have *THE* Wheatstone Bridge, from Wheatstone's lab, as well as some of his notebooks.

You can use a n AC Source for supply and earphones instead of a zero meter. This is how the use the wheatstone bridge in ancient times, before sensitive needle meters where available.

impressive kit

An other very interesting bridge concept is the Kelvin Varley divider but they are not very easy to find. You can use them to measure resistance very precise too, but that is not their normal job. I have an ESI and the king of them all, the Fluke 720. You use them for voltage calibration. Not to calibrate a meter, some people use them for that purpose but the internal resistance is much to high for that (to calibrate 10M meters). You connect two voltage sources and a null meter to them and then you can divide one off them to match the voltage of the other. If they are the same the zero meter reads zero. The beauty is that you can calibrate the 720 to itself and a null meter by feeding it zero volt. If you would use a meter to calibrate the voltage source the alignment and calibration of the voltmeter is the limiting factor. Besides that it loads the circuit . That is why there are meter-calibrators specially made to calibrate meters. I use my KV dividers to check my Datron and Fluke Calibrators against my standard cells with full history. At least I did because the standard cell heater controller died a few months ago and that thing is real voodoo. I will make a new controller from a design made by Jim Williams. It was his first publication. The old one used special selected germanium transistors.

At 22:07 I can spot a 6840 date code on one of the resistors. And at 23.24 I can see a 0471 datecode on the back of the needle meter.

Daven used in pro audio pots in the top mix deck of the 50's to 70's . They are very good rather low noise and very stable.

Beautiful technology, Thanks. 😀

The workmanship is a work of art. We must remember, the 1960's was the height of technological inovation, as the direct result of te US push for the Moon, and developments in military aviation and ICBM production. This is evident in the layout and use of wax coated nylon securing of the wire looms.

Wonderful. Though every time I hear "wheatstone bridge" I think it should be a company that makes breakfast cereal.

Back in College, I've worked on a Wheatstone bridge made in France in the 1890s. It was a beautiful, impressive piece of handcraft with full oak casing and brass contacts. Sadly I can't recall the maker.

We have something similar at work but it's a turns tester. It has big bakelite knobs that give a satisfying 'clunk' when you turn them. It is very old and probably pre dates the sixties.

Integrated test fixture is sweet: given me some ideas.

Hi Dave..!! wuuuuuuuuuu...!! what a good retro machine

You should check out the design in the Boy Electrician for making one of these using nichrome wire. It did require a galvanometer, which was a project in an earlier chapter. The wire was used to make references (basically a wire-would resistor on a wooden spool from thread), and the other side of the bridge was about a yard nichrome wire stretched out on a board with a scale along it. The sliding contact for the scale was a screwdriver. As far as longevity goes, Starrett is still in the same building, I believe going back to 1886.

Who ever made that must have taken great pride in there work.

23:05- I would've mistaken those Daven resistors for "Black Beauty' caps! :)

With the theory I miss the statement "when R1 x Rx = R2 x Rdut than V = 0 " (or when R1 x R3 = R2 x R4) than V = 0 . You replaced it with 'the ratio of the two resistors in the parallel strings'.

It doesn't just go to 11, it goes to 10^11 and then 10^12 for that extra push over the cliff.

It's the skin effect when you have higher voltage Dave, PTF Stand-offs are part of the switch

I Definitely would use Those High precision glass resistors to build something

Back in the 1970's used to use a wheatstone for earth rods when installing generators during the 3day week of PM Edward Heath

Lovely !...cheers.

Probly find these still in use in uni physics labs as well as metrology labs.

Tera-ohms! I don’t even know what that means even with 30+ years of experience at the bleeding edge of technology… That’s just ridiculous… love it…

verry nice!

Ahh the wheatstone bridge is used a lot still. I personally see them used on load cells (some sort of strain gauge is the variable resistor). Fortunately, they sell scale interfaces that do the math and have fancy signal processing, but my guess is that there’s some sort of low-passs or alpha-beta filter and then a bunch of other junk. One manufacturer insisted that the fancy signal processing was needed with motion (they made high speed rotary fillers for filling like a gallon of paint, has to be something where weight is important, the most common type of filling is called overflow and works like a gas pump, there’s also volumetric which uses a volumetric pump which are like a double acting syringe)

Interesting is the special considerations required for accurate measurements of values at the extremes, crazy high & crazy low resistance values. A 4 wire meter is simple by comparison to this thingy.

Yeah, I really liked that, thanks.

I love that kit :), its one of my favorite kit the 1960's to 1980s Ive got a Keithley 148 it uses a Valve on the front end, so if your selling that please do let me know Dave ;) Philip

Awesome.

Big empty case for thermal stability without needing vent holes that could comprise signals.

Ha that Alan key 5/64", almost 2mm, whatever, like you said, it fits.

you could try the 4 wire on the Keithley to get better read.

it probably goes into the peta-ohms range because of the 100x knob numbers.

Dave, Welwyn is pronounced well-in; from someone who used to live in Welwyn Garden City!

Calibrating each range, wow! You would want a carefully controlled environment and do a bunch of measurements at a time.

Trying to find date codes: Stamp on the PSU transformer (possible date code, unsure) is 7218 (seen at 21:05 ), and one of the glass resistors is 6840 (at 28:40 and 23:36 ). Other glass resistor at 23:86 shows 7118. Galvanometer at 23:21 shows 0471 (possible date code). I would assume this was made no earlier than 1971 and possibly sometime in 1972. I didn't see the 1968 code, but I think the resistors are more definitive for dating. It's possible those daven resistors and the mallory capacitor have date codes on the side, but I didn't get a clear look at either. Same with the welwyn range selection resistors at 24:43 Will check flickr and edit this once it's posted on flickr.

interesting! I have an old Australian AWA F240 Noise and Distortion Meter( )and the way it measures noise and the signal level is very similar to this! it's uncanny! i guess they are using Wheatsone bridge in it!

You should look at some combiflex stuff, like razoa

Thanks. I'd like to know more about the model 302 electrometer amplifier module. What odds are that it is a varactor based op-amp? Not a mosfet or jfet but using varicap diode imbalance. Fascinating way to build low-leakage amplifiers!

Thats cool. That is sooo cool. Makes me wish I did something with my life. Lol.

So this bridge is the "To infinity and beyond!" model :)

great video but omg for some reason when you look in the shielded range selection can in the end it sometimes sounds like marv the martian is talking. something is not right with the audio for small periods of time here. maybe its just connotations I have between bridge measurements and that cartoon but damn, it goes back to normal at 26 minutes. that box has weird acoustics

I have a UNI-T UT501A on the way. I wonder why your meter only needs to go to 100V and not higher? I assume its because it has a very good null meter and super sensitive amplification circuitry such that it just doesn't need 1000V

I remember made one years ago when I was studying.😂

How can a resistor switch in the wheatstone bridge not introduce reading errors in multiple orders of magnitude of 10^-12 Ohms? Amazing that this ever could work. Even when brand new. Or am I getting something wrong regarding how resistor contact resistance would impact the reading?

that is some heavy lifting for you :)

What value is the resistor from the tri-ax shield to chassis earth?

5:54 - I'm guessing some of those calibration pots need cleaning!

I have a large collection of electronic components I collected while working for an electronics component manufacturer a few years back. Capacitors, resistors, transistors, etc. Surface mount and through-hole. Thousands of bits, mostly unsorted. If you see this and this is something you would be interested in, please contact me. I will cover the cost of shipping from the States.

Bob's not my uncle, but I'm sure he'd leave me an old box of tricks to tie my brains with.

Those transistors wired as back-to-back diodes. I take it that they wanted to clamp the differential input voltage, and they used transistors because of the much higher reverse leakage of small signal diodes?

That bridge will measure down to single digit ohms.

Красив монтаж.

26:40 The two transistors acting as a back-to-back diode clamp ? Old school transistors as diodes better than actual old school diodes for this application ?

sounds like an electronic version of a see saw.

hey EEVblog, have you heard of George Piggott's electro-levitation experiments?

Could you do a video about Voyagers latest technical bug that was in the news the other week?

What do those two transistors with disconnected collectors infront of the Op amp do?

Keithley made some really cool stuff, just too bad they did not keep up with Fluke to sell the product to the military, schools, factories. etc. 🙄😎 oh i have an old Leader Oscope 🥰 we even have a 1966 Portable Millivolt Potentiometer; Model 8686 (Leeds and Northrup). that was used to test Thermocouples 🤩 Bureau of Naval Weapons, Washington, D. C.

With this nifty thing you can certainly measure every lame electron that would pass your conductor from time to time.

26:30 bjts as diodes? That is new.

There are far too many mentions of Jobbies in this Vid for my liking. In Bonny Scotchland, it has an entirely different meaning! See Billy Connelly!

18:40 Yankee 5-64ths .. 😂😂

Arte

You could have gotten away with your weird Euro hex keys. 5/64" ~ 2mm, with an error of only 16 microns/.0006".